Why there's no sign of a climate conspiracy in hacked emails

The leaking of emails and other documents from the Climate Research Unit (CRU) at the University of East Anglia, UK, has led to a media and political storm. The affair is being portrayed as a scandal that undermines the science behind climate change. It is no such thing, and here's why.

We can be 100 per cent sure the world is getting warmer

Forget about the temperature records compiled by researchers such as those whose emails were hacked. Next spring, go out into your garden or the nearby countryside and note when the leaves unfold, when flowers bloom, when migrating birds arrive and so on. Compare your findings with historical records, where available, and you'll probably find spring is coming days, even weeks earlier than a few decades ago.

You can't fake spring coming earlier, or trees growing higher up on mountains, or glaciers retreating for kilometres up valleys, or shrinking ice cover in the Arctic, or birds changing their migration times, or permafrost melting in Alaska, or the tropics expanding, or ice shelves on the Antarctic peninsula breaking up, or peak river flow occurring earlier in summer because of earlier snowmelt, or sea level rising faster and faster, or any of the thousands of similar examples.

There are many ways, theoretically, to warm a planet. Orbital changes might bring it closer to its star. The star itself might brighten. The planet's reflectivity – albedo – can change if white ice is replaced by darker vegetation or water. Changes in composition of the atmosphere can trap more heat, and so on.

It could even be that Earth isn't really warming overall, just that there has been a transfer of heat from the oceans to the atmosphere.

Researchers have to look at all of these factors. And they have. Direct measurements since the 1970s make it certain, for instance, that neither the sun's fluctuating brightness nor changes in the number of cosmic rays hitting Earth are responsible for the recent warming. Similarly, direct measurements over the past century show that the oceans have warmed dramatically. The planet as a whole is getting warmer.

That leaves the rising levels of greenhouse gases in Earth's atmosphere – which have been directly measured – as the main suspects. Working out how these changes should affect the planet's temperature in theory is extremely complicated. The only way to do it is to plug all the detailed physics into computers – create computer models, in other words. The results show that the only factor that produces anything like the temperature rise seen is the observed increase in greenhouse gases.

How do we know the models aren't wrong? From studies of past climate. To take one example, ice cores drilled from the Antarctic ice-sheet show a surprisingly close correlation between greenhouse gas levels and temperature over the past 800,000 years.

During this time, greenhouse gases have never risen as high or as fast as they are now. That means there is still a lot of uncertainty about the extent of future warming – estimates of the effect of doubling CO2, including all feedback processes, range from 2°C to 6°C. But the big picture is clear.

Is it possible that tens of thousands of scientists have got it wrong? It is incredibly unlikely. The evidence that CO2 levels are rising is irrefutable, and the idea that rising levels lead to warming has withstood more than a century of genuine scientific scepticism.

So why are scientists "fixing" the temperature data?

Some of the contents of the hacked material, such as the "Harry_read_me.txt" file, might appear shocking, with its talk of manipulation and "tricks". But raw data almost always has to be "fixed".

For example, suppose you and your neighbour keep a record of the temperature where you live, and decide to combine your records to create an "official" record for your locality. When you compare records, however, you're surprised to find they are very different.

There are many reasons why this might be so. One or other thermometer might be faulty. Perhaps you placed your thermometer in an inherently warmer place, or where it was sometimes in direct sunshine, or took measurements at a different time of day, and so on. To combine the two records in any meaningful way, you'll need to adjust the raw data to account for any such factors.

Not doing so would be pretty dumb. Where possible, scientists should always look at their data in the context of other, comparable data. Such scrutiny can often reveal problems in the way one or other set of data was acquired, meaning it needs adjusting or discarding. Some apparent problems with the predictions of climate models, for example, have actually turned out to be due to problems with real-world data caused by the failure to correct for factors such as the gradual changes in orbits of satellites.

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The retreat of the Arctic ice is clearly visible in this composite, comparing 2007 levels (top) to 2005 (bottom) (Image: NASA)